(MAINT) Remove RHEL 5 support; normalize OS names

Author: Ciaran McCrisken

metadata.json

@@ -31,6 +31,7 @@
     {
       "operatingsystem": "CentOS",
       "operatingsystemrelease": [
+        "6",
         "7",
         "8"
       ]
@@ -61,6 +62,7 @@
     {
       "operatingsystem": "RedHat",
       "operatingsystemrelease": [
+        "6",
         "7",
         "8"
       ]
@@ -70,7 +72,8 @@
       "operatingsystemrelease": [
         "14.04",
         "16.04",
-        "18.04"
+        "18.04",
+        "20.04"
       ]
     }
   ],